1. Field of the Invention
This invention relates to systems for the chemical vapor deposition of material onto the surfaces of substrates, and more particularly, to a low pressure chemical vapor deposition system having improved means for distributing reactant gases to the substrates and independently evacuating unreacted gases from plenum zones containing substrates to be coated.
2. Prior Art
In one process for making semiconductor substrates (wafers) to be fabricated into integrated circuits (chips), thin films of selected composition and characteristics are deposited onto batches of substrates by chemical vapor deposition at low pressure. Wafers are typically vertically oriented in a batch of 100-200, held in close parallel array supported by slotted quartz rods. The batch is typically removably, concentrically positioned within a longitudinally extending quartz tube reaction chamber which is maintained at a precise temperature and vacuum specific to the chemistry required for the particular thin film product to be deposited.
Vacuum is maintained in the reaction chamber by a mechanical vacuum pump connected to the exhaust end of the system. Two streams of reactants are typically injected into the other (the load) end of the chamber, one stream typically comprised of gases or vapors to provide the cation portion of the deposited thin film as well as dopant cations, the second stream typically comprised of a mixture of gases or vapors to provide the anion portion of the deposited thin film material and dopants.
The injected gases and vapors intermix and flow from the load end points of injection longitudinally along the length of the reaction chamber due to a pressure differential set up by the mechanical vacuum pump. After traveling the length of the chamber, the mixture of gases exits at the exhaust end and flows to the mechanical vacuum pump. During this process, the primary flow path of the constituents of the injected gases or vapors is within the circumferential zone between the inner walls of the reaction chamber and the outer edge of the load of semiconductor substrates. From this primary flow region constituents of the primary stream diffuse perpendicularly into the open spaces between individual substrates and a surface-catalyzed reaction results in a deposit of the desired film onto the substrate surfaces.
Typically, the efficiency of the system is only 15-25%, so that 75-85% of the injected gases and vapors travel along the primary flow path as described above and exit the reactor (as a mixture) to enter the vacuum exhaust line and the mechanical vacuum pump. The exhaust gases and vapors continue to react in the exaust line and vacuum pump, depositing material along the flow pathway. The deposited material is typically in the form of abrasive particles and powdery dusts. In addition, large flakes or chunks of deposited material dislodge from the inner walls of the vacuum exhaust line and are pulled into the mechanical vacuum pump. The large chunks and abrasive particles quickly initiate mechanical abrasion of the internal pump components, thus leading to rapid pump wear-out and failure. The powdery dusts mix with the pump fluid to form a heavy paste that precipitates and blocks fluid flow passages and thus prevents proper pump lubrication. This leads to premature bearing wear and ultimately to freezing of the pump.
Replacement of vacuum pumps is very costly. The pump itself represents an expense, but even more costly is the down-time on the system. The time to reduce the power, remove the failed pump and install a new or rebuilt one, increase the power and requalify the process may take 1-2 operating shifts. In this time, thousands of wafers in process may be delayed, which results in a similar lost time for the equipment performing the next sequential process operation. Vacuum pump replacement is also a very dirty process, and is thus not compatible with the clean room operation of the deposition process.
A trap may be inserted between the pump and the deposition system to help protect the vacuum pump. However, the trap has an adverse effect on the system because its flow properties change constantly as solids are added to it. This leads to a difficulty in controlling the reaction chamber pressure, which in turn leads to difficulty in controlling the deposition process. Furthermore, accumulated material in the trap may find its way back into the reaction chamber due to backstreaming during evacuation and venting cycles and thus contaminate substrate material. This contamination reduces the yield and results in a very costly loss of product. Finally, the trap is often messy and sometimes dangerous to clean out due to the nature of the materials which are dealt with.
Another protective measure which may be used is a step of circulating and filtering the pump fluid with an external filtration system. However, this step is of only limited value since the destructive material is already in the pump and the filtration system can only remove that portion which stays suspended in the fluid, and damage is inflicted by that portion which precipitates out onto the internal pump hardware.
Finally, accumulated material in the exhaust hardware finds its way back into the system reaction chamber during evacuation and vent cycles, contaminates substrates, and results in lowered product yields.